1. Field of the Invention
The invention relates to a system for transmitting optical signals in optical fibers at long distances between locations. In particular, the invention relates to a system operative to monitor and control the integrity of transmission fibers and functionality of the system's components.
2. Prior Art Discussion
The unprecedented growth of communication services has spawned a new era of personal and business interactions. However, such an explosion has brought many challenges to the telecommunication industry to develop technology that will greatly expand the bandwidth limitations of existing communication systems. Optical communications holds great promise to meet the continual demands for greater and greater bandwidth.
In particular, Wavelength Division Multiplexing (WDM) technology, and even more specifically Dense WDM (DWDM), allows for the concurrent transmission of multiple channels over a common optical fiber. The advent of Erbium Doped Fiber Amplifiers (EDFA) has accelerated the development of WDM systems by providing a cost-effective optical amplifier that is transparent to data rate and format. EDFAs amplify all the wavelengths simultaneously, enabling the composite optical signals to travel enormous distances (e.g., up to several hundred kms) without regeneration.
Not surprisingly, the telecommunication services over WDM systems have garnered tremendous attention. Notably, the industry has focused on the design of ultra-long systems (up to thousands of kms) that are capable of transporting high number of channels without optical regeneration.
In addition to EDFAs, Distributed Raman amplification (DRA) provides very high gain across a wide range of wavelengths. DRA, thus, increases the distance between optical regeneration, while allowing for closer channel spacing. The operation of DRA involves inserting high-power laser light in the transmission fiber to amplify the WDM signals. There has been some research in deploying EDFAs in conjunction with DRAs.
As the complexity and power of optical communication systems increase, the demand for additional diagnostics and safety measure associated with the modern systems is being raised as well. It is rather customary that the power of optical signals transmitted through a fiber reaches the order of Watts. The moment there is a fiber break, which can happen due to a number of external and/or internal reasons, light is no more contained within the fiber and poses a safety hazard to field operators dealing with the maintenance of a transmission system at a variety of deployments. Meeting the heightened monitoring and safety requirements, the ITU-T provides guidelines and requirements for techniques enabling optically safe working conditions in the optical transport network. In particular, the ITU-T specifies several classes of safety ranging between safe and hazardous classes 1 and 4, respectively, with a few intermediary subclasses therebetween. In case of a hazardous situation, it is imperative that the optical power be reduced to acceptable safe power levels and even completely shutdown. The monitoring system recommended by ITU-T includes an optical supervisory channel (OSC) representing a particular interest for the scope of this disclosure. The OSC carries management information about the multi-wavelength optical signal as well as remote conditions at the optical terminal or regeneration site, it is also used for remote software upgrades and user (i.e., network operator) Network Management information
Because of the high-power outputs that long-haul application requires, an automatic line shutdown and/or automatic power reduction system, further referred to as the ALS, are provided to automatically reduce the optical power and/or shutdown optical emission. When there is no fiber brake or equipment malfunction, the OSC signal of the OS channel is received at the far end by a media converter confirming, thus, the presence of link. Typically, if the signal of OSC channel is not detected, the system is automatically shut down. Yet, as one of ordinary skilled in the art readily understands, the alarm may be false and the system can quickly restore without being shutdown. Restarting the system may be a complicated process. Accordingly, the system may not be sufficiently robust.
It is, therefore, desirable to provide a long-haul optical transmission system with an automatic line shutdown (ALS) feature capable of at least partially overcoming the above-mentioned drawback of the known systems.
It is further desirable to provide a long-haul optical transmission system with an ALS feature utilizing an EDFA as a switch which operates so as to suppress a customer signal and to transmit only a control signal of in-band OS channel without additional opto-electronic components.
It is also desirable to provide a long-haul optical transmission system with an ALS feature allowing for a useful redundancy of in-band and out-of-band OS channels to discriminate between false and actual alarms and to avoid the automatic shutdown of the transmission system in response to false alarms.
It is further desirable to provide a modular long-haul optical transmission system configured with various combinations of system components that are operative to realize the ALS feature according to the present disclosure.